How to make a walk cycle loop?
If the walk cycle is the only thing you want in your animation, you could just make the timeline the same length as the walk cycle. It'll loop automatically.

Otherwise, if the walk cycle is in your timeline, you can go to the NLA editor with the rig selected (in object mode), and click on the double-down-arrow icon, which will save the walk cycle as an action. 



Then you can use the N key properties window to change the amount of times the action is repeated. 



This is, IMHO, the best solution as it gives you more flexibility to work with animations.